The invention relates to an apparatus for applying a web treatment substance to the surface of a moving web of paper or paperboard by means of spray nozzles.
Currently, application of coating to a web of paper and paperboard is carried out using a plurality of different methods. Each application method has certain characteristic features that affect the quality of the finished product and its manufacturing process. In regard to the optimal production process, the choice must be made based on the actual running speed and strength of the web. The qualities to be rendered to the finished product are determined by the end use of the product, which in practice is defined by the printing method applied to the product and the intended degree of quality to be rendered to the final printed surface. In general, good printability can be achieved by giving the web a sufficiently smooth and even surface having an extremely high brightness. These qualities can be attained by applying a sufficient number of coating layers to the web being processed and then calendering the same in the different phases of paper manufacture. Obviously, as a high number of treatment steps increase the end price of the product, it must be understood that the choice of the proper manufacturing technique is ultimately dictated by the qualities that are desired from the end product. After the suitable process for the desired end product is chosen, it is possible to select such a finishing technique that renders the optimum result in terms of production efficiency and end product quality.
One promising application method is the spray-coating technique, wherein the coating furnish or other web treatment material is applied to the surface of a paper or paperboard web by means of high-pressure spray nozzles. In this method, the web treatment material is pressurized to a high pressure and then sprayed from a small-orifice spray nozzle at a high velocity onto the web. A benefit of the method is that it imposes minimal stress on the web resulting in a good runnability behavior of the spray-coating apparatuses. As it is implemented in a fully noncontacting fashion, this kind of application excels over conventional blade and film-transfer techniques by being more gentle and less stressing to the web thus facilitating a production efficiency increase of about 5% as compared with blade application. In a different point of view, this allows the web to be produced from a stock of weaker strength, which is a particular benefit in making paper from recycle fiber. Contacting application methods such as blade coating or film-transfer coating always need between the web surface and the excess-coating-doctoring element a gap that determines the thickness of the applied coat. Due to this and other factors, the final coat weight is affected by quality variations in the base sheet, among other things by its profile deviations and surface roughness. In contrast, a spray-coater applies to the web under all circumstances a coating layer of constant weight entirely unaffected by base sheet quality variations. Moreover, spray-coating can be performed to a base sheet whose moisture content is higher and, hence, whose strength is lower than what has been acceptable earlier.
Since the coating mixture, surface size or other web treatment substance is applied in the spray-coating technique to the web surface in the form of a droplet shower that flies in a free space the travel from the nozzle tip to the web being treated, a practicable implementation of the system is complicated by the tendency of the coating mist to escape to the surrounding atmosphere. Therefore, the spray nozzles must be enclosed by a hood. In the hood the coating mixture mist condenses on all surfaces wherefrom it must be recovered so that large drops of the condensate are stopped from reaching the web surface. Furthermore, the condensed coating mixture must be prevented from access to the coating spray. While all application methods are hampered by the entry of the air boundary layer traveling along with the moving web to the application zone, this problem appear particularly accentuated in a spray-coater, because the spray of the web treatment substance must penetrate through the air boundary layer by virtue of its kinetic energy alone. Due to the minute size of the coating mixture droplets, also their kinetic energy remains quite small even at a high initial velocity of the sprayed droplets.
Various kinds of assemblies for collecting aerosol mists are known from, e.g., the art of spray-painting arrangements. Since these assemblies are designed, however, for handling unit articles, they permit paint application to be carried out in an entirely closed space or, alternatively, the materials to be treated are so durable and travel at such a slow speed that the space of active treatment of articles can be readily isolated from the surrounding space. These painting or, more broadly, surface treatment systems are used for surface treatment of sheet metal, for instance. In contrast, the web speeds today used in papermaking machines are so high that the air boundary layer traveling along with the web has grown to be an extremely powerful factor of disturbance. Furthermore, the trend is to run as thin base webs as possible made from low-strength fiber in order to cut down the product price. This results in a conflicting situation, wherein the flow of the air boundary layer at the critical areas of the web surface needs to be prevented by effective means but, however, the moving web itself cannot take a robust mechanical contact, for instance.
In U.S. Pat. Nos. 6,106,902 and 6,063,449 are disclosed some techniques for collecting coating mist in spray-coater apparatuses and sealing the applicator chamber. Among these methods deserve to be mentioned, e.g., running the applicator chamber under a vacuum, the use of steam or air flow for controlling the travel of the coating mixture mist and the formation of a film of the web treatment substance flowing along the walls of the applicator chamber.